Usually ceramic substrates such as tiles or sanitary fittings are coated with enamels in order to be highly decorative as well as scratch, wear and solvent resistant. However enameling consumes large amounts of energy and has limitations in view of decoration.
U.S. Pat. No. 4,143,181 relates to a method of applying a coating(s) composed of a primer and a topcoat on glass substrates. The primer, intended to protect the substrate from damage upon impact, is applied as a solution comprising a thermosetting binder composed of a hydroxy functional polyester; the powder topcoat serves for improving resistance to caustic soda and also comprises a thermosetting binder composed of a hydroxy functional polyester.
DE 19748927 relates to a method for obtaining a scratch resistant decorative or functional coating obtained from a thermosetting composition comprising polyester or polyurethane resins on heat resistant non metallic substrates. The substrate is heated to a temperature which is above the softening temperature of the coating to be applied. Subsequently the powder is applied to the heated substrate without the use of electrostatic field, the substrate itself being electrically insulated. The substrate then is transferred to an oven for curing the thermosetting powder. A two coat system is illustrated: a first black powder coating, a polyester TGIC system, is applied at a thickness of 200 microns and cured for 10 minutes. Subsequently a second clear layer of the same polyester TGIC system containing brass flakes is applied and cured for another 10 minutes in order to obtain a highly decorative coating.
WO 2008/055921 relates to a process for coating ceramic substrates with a powder coating composition comprising at least one polyester having carboxy- and/or hydroxy-functional groups and at least one hardener having functional groups reactable with the polyester functional groups as base coat, and with a coating composition comprising a radiation curable resin as further layer.
WO 2008/055922 relates to a process for coating ceramic substrates with a powder coating composition comprising at least one acrylic copolymer comprising carboxy hydroxy and/or glycidyl functional groups and at least one hardener having functional groups reactable with the acrylic copolymers' functional groups to the substrate. A powder primer layer is often first applied.
U.S. Pat. No. 6,982,137 relates to a method of forming color images on tiles or glass wherein the substrate is first coated with a clear powder polymer coating cured at 80% to 95%, then a xerographically color image is applied, finally another layer of the same polymer and then the system is heated to achieve complete cure.
None of these prior art coatings obtain a high decorative finish with outstanding mechanical and chemical performance, such as scratch resistance and chemical resistance.
Hardness of the finishes proposed thus far was often not sufficient, and especially extreme hardness (3H-4H) in combination with outstanding solvent and heat shock resistance formed a technical challenge.